1. Field of Invention
This invention relates in general to electrophotographic imaging members and more specifically to an improved electrophotographic imaging member having a charge generation section comprised of layers of two different photoconductive pigments. The charge generation section is comprised of a layer of hydroxygallium phthalocyanine photoconductive pigment and a layer of benzimidazole perylene photoconductive pigment.
2. Description of Related Art
In the art of electrophotography, an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the imaging surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated area. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable electrophotographic imaging members.
The electrophotographic imaging members may be in the form of plates, drums or flexible belts. These electrophotographic members are usually multilayered photoreceptors that comprise a substrate, a conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, an optional overcoating layer and, in some belt embodiments, an anticurl backing layer. One type of multilayered photoreceptor comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. In U.S. Pat. No. 4,265,990 a layered photoreceptor is disclosed having separate charge generating (photogenerating) sections and charge transport layers. The charge generation section is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer.
The charge generating section utilized in multilayered photoreceptors include, for example, inorganic photoconductive particles or organic photoconductive particles dispersed in a film forming polymeric binder. Inorganic or organic photoconductive material may be formed as a continuous, homogeneous charge generation section. Many suitable photogenerating materials known in the art may be utilized, if desired.
Electrophotographic imaging members or photoreceptors having varying and unique properties are needed to satisfy the vast demands of the xerographic industry. The use of organic photogenerating pigments such as perylenes, bisazos, perinones, and polycyclic quinones in electrophotographic applications is well known. Generally, layered imaging members with the aforementioned pigments exhibit acceptable photosensitivity in the visible region of the light spectrum, and hence they are particularly suitable for use in electrophotographic processes where visible light sources such as tungsten, fluorescent, and xenon lamps are used.
However, these classes of pigments in many instances have low or negligible photosensitivity in the near infrared region of the spectrum, for example between about 750 and 970 nanometers, thereby preventing their selection for photoresponsive imaging members in electronic printers wherein electronic light emitting devices, such as GaAs diode lasers, are commonly used as a light source to create an electrostatic image on the imaging members. Also, some of the above mentioned organic pigments have a narrow and restricted spectral response range such that they cannot reproduce certain colors present in the original documents, thus resulting in inferior copy quality.
To satisfy these demands, photoreceptors with different charge generation section formulations providing varying photo-sensitivities may be utilized. Charge generation sections are often formed by layering a dispersion of photoconductive pigments on to the photoreceptor. The cost to develop different photoconductive pigments and different charge generation section coating dispersion formulations and to change dispersion solutions for different products in the manufacturing process greatly increases the costs to manufacture photoreceptors.
The process of making a photoreceptor using dispersions is strongly susceptible to many variables, such as: materials variables, including contents and purity of the material; process variables, including milling time and milling procedure; and coating process variables, including web coating, dip coating, the drying process of several layers, the time interval between the coatings of successive layers etc. The net outcome of all these variables is that the electrical characteristics of photoreceptors may be inconsistent during the manufacturing process.
Sensitivity is a very important electrical characteristic of electrophotographic imaging members or photoreceptors. Sensitivity may be described in two aspects. The first aspect of sensitivity is spectral sensitivity, which refers to sensitivity as a function of wavelength. An increase in spectral sensitivity implies an appearance of sensitivity at a wavelength in which previously no sensitivity was detected. The second aspect of sensitivity, broadband sensitivity, is a change of sensitivity (e.g., an increase) at a particular wavelength previously exhibiting sensitivity, or a general increase of sensitivity encompassing all wavelengths previously exhibiting sensitivity. This second aspect of sensitivity may also be described as change of sensitivity, encompassing all wavelengths, with a broadband (white) light exposure. A common problem encountered in the manufacturing of photoreceptors is maintaining consistent spectral and broadband sensitivity from batch to batch.
A conventional technique for coating cylindrical or drum shaped photoreceptor substrates to form charge generation sections involves dipping the substrates in coating baths. The bath used for preparing charge generation sections is prepared by dispersing photoconductive pigment particles in a solvent solution containing a film forming binder. Unfortunately, some photoconductive pigments cannot be applied by dip coating and still obtain high quality photoconductive coatings due to settling, shear thinning, etc. in the solvent solution and other problems associated with dip coating.
Some pigments tend to settle in the solvent solution of the film forming binder. This may cause a lower than expected amount of photoconductive pigment to be dispersed onto the charge generation section and thus affect the sensitivity of the coated web or other substrate to be coated. Attempting to offset the tendency to settle requires constant stirring which may lead to the entrapment of air bubbles. Such air bubbles may be carried over into the final charge generation section deposited on a photoreceptor substrate resulting in defects in print quality and/or non-uniform charge generation sections. The settling of the pigments may also result in pigment agglomerates which likewise may lead to defects in print quality and/or non-uniform charge generation sections. The settling of the pigments may also cause streak surface coating defects in the charge generation section through the depositing of pigments in a concentration level other than a desired concentration level in localized portions of the charge generation section.
Shear thinning is another common problem in the development of charge generation sections. Shear thinning occurs when forces of varying magnitudes are applied to a non-Newtonian solution resulting in disparate changes in the nature of the non-Newtonian solution. Newtonian solutions are preferred for dip coating since uniform results in the charge generation section are more likely to occur.
Typically, flexible photoreceptor belts are fabricated by depositing the various layers of photoactive coatings onto long webs which are thereafter cut into sheets. The opposite ends of each photoreceptor sheet are overlapped and ultrasonically welded together to form an imaging belt. In order to increase throughput during the web coating operation, the webs to be coated have a width of twice the width of a final belt. After coating, the web is slit lengthwise and thereafter transversely cut into predetermined lengths to form photoreceptor sheets of precise dimensions that are eventually welded into belts. The web length in a coating run may be many thousands of feet long and the coating run may take more than an hour for each layer.
The coating solution may be kept in a pressure pot prior to and during application. The manufacturing of multi-layered photoreceptors containing perylene pigment dispersion in the charge-generating layer may require several hours. In general, photoconductive pigment loadings of 80 percent by volume in a binder resin or a mixed resins binder are highly desirable in the charge generation section to provide excellent photosensitivity. However, these dispersions are highly unstable to extrusion coating conditions, resulting in numerous coating defects that generate a large amount of unacceptable material that must be thrown away when using extrusion coating with a dispersion of pigment in an organic solution of polymeric binder. More stable dispersions can be obtained by reducing the pigment loading to 30-40 percent by volume, but in most cases the resulting xe2x80x9cdilutedxe2x80x9d photogenerating layer does not provide adequate photosensitivity. Also, the dispersions of higher pigment loadings generally provide a photoreceptor layer with poor to adequate adhesion to either the underlying ground plane or adhesive layer, or the overlying transport layer when polyvinylbutyral binders are utilized in the charge generation section. Many of these organic dispersions are quite unstable with respect to pigment agglomeration, resulting in dispersion settling and the formation of dark streaks and spots of pigment during the coating process.
A need to increase the sensitivity of a photoreceptor may also exist without the aforementioned potential causes of change in sensitivity. A photoreceptor with only BZP may not provide sufficient sensitivity and a photoreceptor with a higher sensitivity may be desired.
Previous attempts to overcome the aforementioned problems associated with dip coating have led to the development of a charge generation section containing a mixture of two different pigment dispersions comprising:
(1) titanyl phthalocyanine (TiOPC) and
(2) chloro indium phthalocyanine (ClInPC).
See, for example, U.S. Pat. No. 5,418,107. Both pigments are dispersed within polyvinyl butyral binder in n-butyl acetate (nBuOAc) solvent. These two dispersions have different sensitivities. By mixing different ratios of these two dispersions, different levels of photosensitivity may be achieved enabling the manufacture of different photoreceptors with varying charge generation layers. However, this mixture of TiOPC and ClInPC has proven unstable and results in streaking in the prints.
The present inventors have found that the stability problem results mostly from the ClInPC dispersion. The ClInPC exhibits strong shear thinning behavior at higher solids (e.g., 6% by weight). Although the ClInPC dispersion becomes Newtonian after being diluted down to about 3%, it still settles upon sitting for a few days. The settling of the ClInPC dispersion is likely caused by the low viscosity of the solution and agglomeration of the ClInPC dispersion.
The above-mentioned U.S. Pat. No. 5,418,107 thus describes a photoconductive layer comprised of a mixture of at least two different phthalocyanine pigments free of vanadyl phthalocyanine pigment particles. The selected pigment particles have an average particle size of less than about 0.6 micrometers and preferably less than about 0.4 micrometers. Typical mixtures of photoconductive particles include metal-free phthalocyanine and titanyl phthalocyanine, chloro indium phthalocyanine and titanyl phthalocyanine, and hydroxygallium phthalocyanine and titanyl phthalocyanine. Satisfactory results are achieved when the selected pigment particles comprise about 50-90% by weight of the dried photoconductive layer, with each of the individual pigments comprising at least about 5% of the total weight of the pigment. The pigments are dispersed in a solution of a film forming polyvinyl butyral dissolved in an alkyl acetate solvent. The use of perylene pigments is not taught by the examples and embodiments of this reference. The reference in fact teaches that the use of benzimidazole perylene pigments leads to settling, thus causing poor results in xerographic printing (column 1, lines 43-50).
U.S. Pat. No. 4,882,254 describes a photoconductive layer comprised of a mixture of photoconductive pigments providing a varied spectral response depending on the mixture of photoconductive pigments selected. The photoconductive pigments include metal phthalocyanines, or metal free phthalocyanines with quinacridones, perylenes, anthanthrones, perinones, pyranthrones, indogoides and bisazos. A preferred embodiment uses a pigment mixture of BZP and vanadyl phthalocyanine. The conductive pigments selected are utilized in a ratio of 10-90% of the first pigment and 90-10% of the second pigment.
U.S. Pat. No. 5,725,985 describes an electrophotographic imaging member having a charge generation layer comprised of photoconductive particles of hydroxygallium phthalocyanine and titanyl phthalocyanine dispersed in a polymer matrix of a film forming terpolymer reaction product and a film forming copolymer reaction product. The film forming terpolymer reaction product results from vinyl chloride, vinyl acetate and maleic acid. The film forming copolymer reaction product results from vinyl chloride and vinyl acetate. The photoconductive particles are present in an amount of about 50% to about 65% by weight of the charge generation layer with an optimal amount identified as 60% by weight. The relative amounts of hydroxygallium phthalocyanine and titanyl phthalocyanine are not disclosed.
U.S. Pat. No. 5,571,647 describes an electrophotographic imaging member comprised of a support substrate having a two layered electrically conductive outer surface, a charge generation layer comprised of photoconductive particles of perylene or phthalocyanine dispersed in a film forming resin binder blend of polyvinyl butyral polymer and one or two copolyesters. The perylenes may comprise between about 20% and about 90% of the total volume of the dried charge generating layer. Optimum results are obtained when the perylenes comprise about 35% to about 45% by volume. It is not disclosed that the photoconductive particles may be mixed.
U.S. Pat. No. 5,863,686 describes an electrophotographic imaging member comprised of a supporting substrate, an undercoat layer doped with a donor molecule, a charge transport layer and a charge generation layer. The donor molecule donates an electron to a photoconductive pigment when it is exposed to light. Benzimidazole perylene and dibromoanthrone are described as being known photoconductive particles for use in the charge generation layer. It is further described that benzimidazole perylene dispersed in a polyvinyl butyral film forming binder in combination with the donor molecule dissolved in the polyvinyl butyral film forming binder leads to dramatic improvements in sensitivity. It is not disclosed that the photoconductive particles may be mixed.
U.S. Pat. No. 5,521,306 describes a process for preparation of a Type V hydroxygallium phthalocyanine comprising the in situ formation of an alkoxy-bridged gallium phthalocyanine dimer, hydrolyzing the dimer to hydroxygallium phthalocyanine and subsequently converting the hydroxygallium phthalocyanine product obtained to a Type V hydroxygallium phthalocyanine.
U.S. Pat. No. 5,552,253 describes a photoreceptor comprising at least two photoconductive stacks. Each photoconductive stack contains a charge generator layer and charge transport layer. The photoconductive stacks are sensitive to different wavelengths to allow selective discharge for a particular wavelength of light. The reference does not teach the use of benzimidazole perylene and hydroxygallium pthalocyanine together in the same charge generator layer, nor the use of these pigments in adjacent generator layers (since a charge transport layer would separate the layers), and thus does not teach the enhancement of sensitivity obtained by mixing the photoconductive pigments.
U.S. Pat. No. 5,322,755 describes a layered photoconductive imaging member comprising a supporting substrate, a photogenerator layer comprising perylene photoconductive pigments dispersed in a resin binder mixture comprising at least two polymers, and a charge transport layer. The resin binder can be, for example, a mixture of polyvinylcarbazole and polycarbonate homopolymer or a mixture of polyvinylcarbazole, polyvinylbutyral and polycarbonate homopolymer or a mixture of polyvinylcarbazole and polyvinylbutyral or a mixture of polyvinylcarbazole and a polyester. Although improvement in photosensitivity and adhesion are achieved, charge deficient spots print defects can still be a problem. Thus, there is a continuing need for improved photoreceptors that exhibit freedom from charge deficient spots and are more resistant to layer delamination during slitting, grinding, buffing, polishing, and dynamic belt image cycling.
U.S. Pat. No. 5,473,064, describes a process for the preparation of Type V hydroxygallium phthalocyanine, essentially free of chlorine, whereby a chlorogallium phthalocyanine pigment precursor is prepared by reaction of gallium chloride with 1,3-diiminoisoindoline in a solvent such as N-methylpyrrolidone; hydrolyzing said pigment precursor chlorogallium phthalocyanine by, for example, dissolving the pigment precursor in concentrated sulfuric acid, and then reprecipitating in a solvent, such as water, or a dilute ammonia solution; and subsequently treating the resulting hydroxygallium phthalocyanine with a solvent, such as N,N-dimethylformamide, by for example, ball milling said hydroxygallium phthalocyanine pigment in the presence of spherical glass beads. The Type V hydroxygallium phthalocyanine obtained from the chlorogallium phthalocyanine precursor prepared according to this procedure contains very low levels of residual chlorine of from about 0.001 percent to about 0.1 percent of the weight of the Type V hydroxygallium pigment as determined by elemental analysis and can enable improved electrical performance of the Type V hydroxygallium as a photogenerating pigment, and improved desirable dark decay and cycling characteristics for the resulting photoconductive imaging member.
What is still desired is an electrophotographic imaging member that avoids problems of the prior art discussed above and that is comprised of at least two layers of photoconductive pigments exhibiting stable properties when applied to a photoreceptor using a solvent solution of a film forming binder.
It is an object of the present invention to provide an improved photoreceptor having high quality photoconductive coatings which overcomes the above-noted deficiencies. It is another object of the invention to provide for stable pigment dispersions for use in photoreceptors. It is yet another object of the invention to maximize sensitivity in a fixed narrow wavelength band and in the near infra-red wavelength region. It is yet another object of the invention to maximize sensitivity over a broadband of exposure.
These and other objects of the present invention are achieved by providing an electrophotographic imaging member comprising a charge generation section including separate layers of photogenerating particles of:
(1) benzimidazole perylene (BZP), and
(2) hydroxygallium phthalocyanine (HoGaPC).